There are many waste materials that are generated during the production of steel and in other manufacturing processes. During the production of steel, electric arc furnace (EAF) dust and basic oxygen furnace (BOF) dust/sludge are captured and collected for either disposal or recycle.
The major components in these dusts are iron (up to 60 wt. %) and zinc (up to 30 wt. %), usually in the oxide form. In addition, these dusts contain smaller quantities of calcium, magnesium, manganese, chloride, lead, cadmium and other trace elements. The steel industry generates about 30 to 40 pounds of these types of dust for each ton of steel produced in steel making facilities. The estimated generation of these dusts in the United States in 1997 was in excess of one million tons with nearly half of it being disposed in landfills.
EAF dust is a listed hazardous waste (K061) under United States environmental regulations. Under this regulatory program, EAF dust is subject to specific record keeping, handling requirements and processing costs when it is recycled or disposed of. BOF sludge/dust is not a listed hazardous waste, but the lack of adequate reuse options results in a large quantity of this material being disposed of in landfills.
Both EAF dust and BOF sludge/dust contain significant quantities of iron that make these materials valuable for use directly in steel making processes as a substitute for other iron containing materials such as scrap steel, hot metal, sinter dust, and other iron oxides present at steel making facilities. In addition, EAF dust also contains significant quantities of zinc which makes it valuable as a feed for zinc manufacturing processes. BOF sludge/dust contains smaller quantities of zinc, but the zinc present can be recovered for reuse as a zinc product.
Present methods for handling EAF dust include processing it in Waelz kilns, rotary hearth furnaces, and flame reactor processes to recover zinc. EAF dust is also subject to stabilization processes followed by landfill disposal. A small quantity of BOF sludge/dust is returned to the BOF steel making process in some locations and some is recycled in other manners, but a large volume is disposed as a waste material in landfills.
Generally, it can be economical to subject EAF dust having a high zinc content to existing zinc recovery procedures, and send EAF dust having a low zinc content to landfills for disposal as a waste. Since hot metal is the only energy source in BOF facilities, only a limited quantity (approximately 20% of the total steel production rate) of scrap and other solid feed materials that require energy for their use in the BOF process can be added to the charge mix. This limits the quantity of BOF sludge/dust that can be returned to the BOF steel making process. As a result some portion of the BOF sludge/dust is usually sent to landfills for disposal.
Other process methods for recovering zinc and/or iron from EAF dust and BOF sludge/dust have been attempted with limited success. Such processes include thermal processes, hydrometallurgical processes or combinations thereof. In most cases, these processes were not successful because they are expensive to build, expensive to operate and/or they cannot be used to process all of the EAF dust and BOF sludge/dust material, so that waste material remains that still must be disposed of in landfills.
In addition, any processes that treat EAF dust are subject to significant environmental regulatory requirements under the Resource Conservation and Recovery Act (“RCRA”). This is due to the fact that current processes have been specifically developed for the sole purpose of processing EAF dust as a hazardous waste. To date, those in the steel industry view EAF dust as a listed hazardous waste, and accordingly logically conclude that using EAF dust as any type of feed or reactant will necessarily result in the formation, generation or preservation of a hazardous material.
Southwick, “Recovery of Iron and Zinc From Steel Mill Wastes,” a presentation from a Conference Organized by Gorham/Inertech in Pittsburgh, Pa. on May 17-19, 1999 provides a summary of various processes proposed to recover iron and zinc from mill wastes. In addition, U.S. Pat. Nos. 4,605,435, 5,013,532, 5,082,493, 5,435,835, 5,439,505, 5,493,580, 5,538,532, 5,667,553, 5,879,617, 5,993,512, 6,102,982, 6,120,577, and 6,221,124 are directed to various methods and apparatus for processing EAF dust.
U.S. Pat. No. 6,136,059 to Zoppi discloses the use of an induction furnace for the sole purpose of processing EAF dust. That is, the only feed into the induction furnace after providing a “heel” for the required oxidation and reduction reactions is a half charge of cast iron which is not replenished, but rather to which pellets of EAF dust is added together with coal and small amounts of slagging agents. Zoppi mentions that induction furnaces of the prior art are generally only used as a smelting means in secondary steel and non-ferrous metals processing.
U.S. Pat. No. 6,831,939 to Bratina and Fehsenfeld discloses an expansion of the Zoppi patent that involves operating an induction furnace to both produce hot metal and a pig iron product and to process iron and volatile metal containing materials to recover iron value and concentrate metals which are volatile at the operating temperature of the induction furnace. The process relies upon mixing feeding both metal oxides and reduction material into the induction furnace. The efficiency of the process is limited by the induction furnace that is used.
Other patents that discuss the use of induction furnaces in smelting processes that involve steel processing dust include U.S. Pat. Nos. 5,980,606, 5,304,230, 5,249,198, 5,188,658, 4,878,944, 4,802,919, 4,762,554, 4,612,041, and 4,403,327.
The present invention is directed a method of configuring and operating an induction furnace for processing waste materials while producing metal products and recovering iron and volatile metals in a manner that is energy efficiency and provides improved separation of volatile metals and improved control of slag viscosity.